Fiber bundles are used in a wide variety of optical applications including optical fiber amplifiers, couplers (single mode), and splitters. Typical fiber bundles include multi-moded fibers that are bundled and epoxied into a ferrule. This type of fiber bundle is often used as a coupler in a cladding-pumped fiber laser. In this application, light from a plurality of low brightness light sources is received by the fiber bundle coupler and bulk optics are used to couple the light from the fiber bundle coupler into a cladding-pumped fiber. See for example, U.S. Pat. No. 5,268,978.
Each multiple-moded fiber of the fiber bundle coupler may include a pure silica core surrounded by a silica cladding doped with fluorine. The fiber bundle couples light received from low brightness light sources into the innermost cladding of a cladding-pumped fiber. The light in the cladding interacts with the core of the pumped fiber and is absorbed by a rare-earth dopant in the core. If an optical signal is passed through the pumped core, it will be amplified; if optical feedback is provided, the cladding-pumped fiber will act as a laser oscillator at the feedback wavelength.
More recently, fiber bundles have been constructed into tapered multiple-moded fiber bundles which are used for more efficiently coupling stripe emitter laser diodes into the cladding of a cladding-pumped fiber. A tapered multiple-moded fiber bundle coupler typically includes a plurality of multiple-moded fibers which extend from an unbundled end region and converge together into a cylindrical-shaped fused bundled intermediate region. The intermediate region, in turn, tapers to a reduced diameter region that approximates the diameter of the cladding-pumped fiber. Such couplers are disclosed in U.S. patent application Ser. No. 08/897,195. Now U.S. Pat. No. 5,864,644 entitled TAPERED FIBER BUNDLES FOR COUPLING LIGHT INTO AND OUT OF CLADDING-PUMPED FIBER DEVICES, filed on Jul. 21, 1997 by the present inventor DiGiovanni et al., which is incorporated herein by reference. At the unbundled region of the coupler, the end of each fiber is coupled to one of the single strip emitter laser diodes. The reduced diameter region of the coupler is coupled to the innermost cladding layer of the cladding-pumped fiber.
A tapered multiple-moded fiber bundled coupler allows the use of a cladding-pumped fiber with a suitably increased numerical aperture which theoretically causes no loss of power. Accordingly, a tapered multiple-moded fiber bundle coupler in conjunction with existing cladding-pumped fiber laser technology allows the fabrication of a fiber laser with reliability much less dependent on the pump diodes.
Tapered multiple-moded fiber bundled couplers are presently fabricated using a transverse fusing process developed for the fabrication of single mode, fused fiber bundle couplers. Single mode couplers typically employ two or three fibers which are twisted together and put under tension. Heat is then applied orthogonal to the axes of the fibers using a small flame or electrical element, and the diameters are reduced by up to 80 percent. Multiple-moded fiber bundled couplers, in contrast, employ more fibers (typically seven) whose diameters are each reduced by less (typically 30 to 50 percent). This lesser diameter reduction combined with the greater number of fibers, results in more than 150 times the cross-sectional area being heated. As consequence, small flames or heater elements employed in the fabrication of single mode couplers can not achieve the heat necessary to melt the extra volume of silica present in tapered multiple-moded fiber bundle couplers. Further, this large mass of silica produces across the fibers a large temperature gradient which often results in deformation during diameter reduction.
Larger flames have been used to generate the necessary heat to fuse the fibers of a tapered multiple-moded fiber bundle coupler, but do so with higher gas flow producing a different type of deformation. The high gas flow against the fibers produces diameter oscillations and/or bending during fusion. In an attempt to eliminate the fiber bending, multiple torches oriented so that they exert opposing forces on the fibers have been used. However, this method requires careful control over gas flow rates at each torch, identical torch design, precise torch location, and shielding from ambient air currents. Baffles or ovens heated by torches have also been used but they typically produce too little heat to allow rounding of the fibers.
The epoxied fiber bundles described earlier have reliability problems and, gaps exist between the bundled fibers that degrade their brightness. Thus, it is also desirable to make these fiber bundles with discrete fibers that are fused and rounded into cylinders or other geometrical shapes for use in various applications. The fabrication of these bundles also requires high temperatures produced by large transverse flames which produce deformation problems.
Accordingly, there is a need for improved method of fabricating a fused fiber bundle which avoids the problems associated with transverse fusing.